Polyanhydroaspartic acid (also known as polysuccinimide) is the key commercial precursor to polyaspartic acid, an increasingly important biodegradable water soluble polymer. The chemistry and process fundamentals of the preparation of polysuccinimide by the thermal condensation polymerization of aspartic acid are well established in the patent and journal literature. Illustrative of such methods are those described in U.S. Pat. No. 5,057,597, to Koskan and U.S. Pat. Nos. 5,315,010 and 5,221,733 to Koskan et al. The weight average molecular weights obtained by thermal condensation of aspartic acid by these methods are usually about 5000.
Additives to the aspartic acid monomer have also been employed to obtain higher weight average molecular weight polysuccinimide. The two most commonly used additives are concentrated phosphoric acid as reported in Neri, et. al., J. Medicinal Chemistry, 16, 893-897 (1973) and its cited references and phosphoric anhydride as reported by Knebel, et. al. U.S. Pat. No. 5,142,062. Detailed descriptions concerning the mechanism by which these phosphoric acid based additives work have not been widely published in the literature. Some references note their acidic abilities while others note their dehydrating abilities.
There are no commercial processes employing phosphoric acid based additives to prepare higher weight average molecular weight polysuccinimide for at least two reasons. First the cost of the reactants, phosphoric acid and its anhydride, are relatively high. Second, waste streams containing phosphate salts are damaging to the environment because phosphorous promotes algae blooms in lakes and rivers which deplete the level of oxygen necessary to sustain marine life.
The present invention provides an ecologically tolerable phosphoric acid free and economical manufacturing process for preparing high weight average molecular weight polysuccinimide which overcomes these disadvantages.